Lithium manganese compounds and methods of making the same

ABSTRACT

Electrode materials such as Li x MnO 2  where 0.2&lt;x≦2 compounds for use with rechargeable lithium ion batteries can be formed by mixing LiMn 2 O 4  compounds or manganese dioxide compounds with lithium metal or stabilized and non-stabilized lithium metal powders.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 60/695,159, filed Jun. 29, 2005, the disclosure of which isincorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention generally relates to methods for forming lithiumcompounds, and the compounds formed by such methods. More particularly,this invention relates to methods for forming lithium manganesecompounds and doped lithium manganese compounds by lithiationtechniques.

BACKGROUND OF THE INVENTION

Attractive materials for use as cathode materials for lithium ionsecondary batteries include LiCoO₂, LiNiO₂, and LiMn₂O₄. Unlike LiCoO₂and LiNiO₂, the LiMn₂O₄ spinel compounds are believed to be overchargesafer and are desirable cathode materials for that reason. Nevertheless,although cycling over the full capacity range for pure LiMn₂O₄ can bedone safely, the specific capacity of LiMn₂O₄ is low. Specifically, thetheoretical capacity of LiMn₂O₄ is only 148 mA·hr/g and typically nomore than about 115-120 mA·hr/g can be obtained with good cycleability.LiMn₂O₄ can contain excess lithium on the 16d manganese sites and can bewritten as Li_(1+x)Mn_(2−x)O₄ (0≦x≦0.33). Use of the formula LiMn₂O₄herein is understood to denote Li_(1+x)Mn_(2−x)O₄ as well.

The orthorhombic LiMnO₂ and the tetragonally distorted spinel Li₂Mn₂O₄have the potential for larger capacities than those obtained with theLiMn₂O₄ spinel. However, cycling over the full capacity range for LiMnO₂and Li₂Mn₂O₄ results in a rapid capacity fade. Layered LiMnO₂ quicklyconverts to a spinel form upon cycling which also results in a capacityfade.

Various attempts have been made to either improve the specific capacityor safety of the lithium metal oxides used in secondary lithiumbatteries by doping these lithium metal oxides with other cations. Forexample, U.S. Pat. No. 6,214,493 to Bruce et al. relates to stabilizedlayered LiMnO₂ using cobalt (Co) as a dopant material. Stabilization hasbeen recorded with as little as 15 percent cobalt substitution. Inanother example, U.S. Pat. No. 5,370,949 to Davidson et al. proposesthat introducing chromium cations into LiMnO₂ can produce a tetragonallydistorted spinel type of structure which is air stable and has goodreversibility on cycling in lithium cells.

Li₂MnO₂ compounds have also been considered as electrode materials. U.S.Pat. No. 4,980,251 to Thackeray proposes that Li₂MnO₂ can be formedhaving a theoretical capacity of 530 mA·hr/g by reacting LiMn₂O₄ spinelcompounds with n-BuLi as follows:LiMn₂O₄+n-BuLi→Li₂Mn₂O₄+2n-BuLi→2Li₂MnO₂The Li₂MnO₂ has a hexagonal close packed layered structure, similar tothe structure of LiCoO₂, except that the Li⁺ ions in Li₂MnO₂ occupy thetetrahedral sites instead of the octahedral sites as in LiCoO₂. However,the Li₂MnO₂ compounds formed according to Thackeray's methods areunstable. In particular, Thackeray notes that the layered structure ofLi₂MnO₂ is unstable and that it converts back to the spinel frameworkupon delithiation. This is undesirable because repeated conversionbetween layered and spinel structures decreases capacity retention andresults in voltage gaps.

A doped lithium manganese oxide preferably exhibits a high usablereversible capacity and good cycleability to maintain reversiblecapacity during cycling. LiMn₂O₄ can generally only be operated at115-120 mA·hr/g with good cycleability. Furthermore, Li₂MnO₂ compoundsare expensive to make and are unstable when made according to availablemethods. Therefore, there is a need to produce a lithium metal oxidethat exhibits an improved reversible capacity and good cycleabilitywhile maintaining thermal stability.

SUMMARY OF THE INVENTION

Embodiments of the present invention include methods for making lithiummanganese oxide compounds and doped lithium manganese oxide compounds.The lithium manganese compounds and doped lithium manganese oxidecompounds formed according to embodiments of the present invention canbe used to form electrodes and electrode materials for use in batteries,such as rechargeable lithium ion batteries.

According to some embodiments of the present invention, a doped lithiummanganese spinel compound is mixed with lithium metal to produce a dopedLi_(x)MnO₂ compound where 0.2<x≦2. The mixing of the spinel compound andlithium metal can be performed with or without a solvent. Mixing of thespinel compound and lithium metal can be performed using processescapable of energetically mixing the doped lithium manganese spinelcompound and lithium metal, such as by high energy ball milling. Themixing preferably provides as much contact between the spinel compoundand the lithium metal as possible. A doped lithium manganese spinelcompound can include compounds such as those disclosed by U.S. Pat. No.6,267,943 to Manev et al., which is incorporated in its entirety hereinby reference. The lithium metal is preferably a stabilized lithium metalpowder such as those disclosed by U.S. Pat. Nos. 5,567,474 and 5,776,369to Dover et al., which are incorporated herein by reference in theirentireties. One of the added advantages of the present invention is thatthe amount of lithium x in Li_(x)MnO₂, where 0.2<x≦2, can be easilycontrolled and varied by varying the amount of the lithium metal used insynthesis, unlike high temperature solid state synthesis where the xvalue is governed by the high temperature phase diagram and may not bechanged at will.

In other embodiments of the present invention, a manganese dioxide suchas a heat treated electrolytic manganese dioxide (EMD) compound can bemixed with a lithium metal to lithiate the manganese dioxide compound.The lithiated manganese dioxide such as the lithiated EMD material canbe used as an electrode material in rechargeable lithium ion batteries.The lithium metal powder is preferably a stabilized lithium metal powdersuch as those disclosed by U.S. Pat. Nos. 5,567,474 and 5,776,369 toDover et al.

Electrodes for use in batteries, and particularly for use withrechargeable lithium ion cell batteries, can be formed using theLi_(x)MnO₂ where 0.2<x≦2 compounds or lithiated EMD materials formedaccording to embodiments of the present invention.

The foregoing and other aspects of the present invention are explainedin greater detail in the specification set forth below and will beapparent from the description of the invention and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphic comparison of x-ray diffraction patterns accordingto Example 1.

FIG. 2 is a graphic comparison of x-ray diffraction patterns accordingto Example 2 and Comparative Example 1.

FIG. 3 is a graph of Voltage (V) versus Specific Capacity (mAH/g)relating to Example 2.

FIG. 4 is a graphic comparison of x-ray diffraction patterns accordingto Examples 3 and 4, and Comparative Example 1.

FIG. 5 is a graph of Voltage (V) versus Specific Capacity (mAH/g)relating to Example 3.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter. Thisinvention may, however, be embodied in many different forms and shouldnot be construed as limited to the embodiments set forth herein; rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the invention to thoseskilled in the art.

The terminology used in the description of the invention herein is forthe purpose of describing particular embodiments only and is notintended to be limiting of the invention. As used in the description ofthe invention and the appended claims, the singular forms “a”, “an” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. Additionally, as used herein, theterm “and/or” includes any and all combinations of one or more of theassociated listed items.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs.

All publications, U.S. patent applications, U.S. patents and otherreferences cited herein are incorporated by reference in theirentireties.

Embodiments of the present invention include methods for making lithiummanganese oxide compounds and doped lithium manganese oxide compounds.The lithium manganese compounds and doped lithium manganese oxidecompounds formed according to embodiments of the present invention canbe used to form electrodes and electrode materials for use in batteries,such as rechargeable lithium ion batteries.

According to embodiments of the present invention, methods for forming alithium manganese oxide compound having the formula Li_(x)MnO₂ where0.2<x≦2 are provided. In some embodiments, the lithium manganese oxidecompound can be a doped lithium manganese oxide compound. For example, adoped lithium manganese oxide compound having the formulaLi₂Mn_(1−α)A_(α)O₂ can be formed, wherein A is a dopant and 0≦α≦0.5.

A lithium manganese oxide compound having the formula Li_(x)MnO₂ where0.2<x≦2, often 0.5<x≦2 can be formed according to embodiments of thepresent invention by mixing an LiMn₂O₄ spinel compound with lithiummetal. As the LiMn₂O₄ compound comes in contact with the lithium metal,the compound accepts the lithium and converts to the desired Li_(x)MnO₂compound. For example, an LiMn₂O₄ compound can be mixed with lithiummetal in a ball mill to form Li_(x)MnO₂. The lithium metal is preferablya stabilized lithium metal powder. The mixing of the LiMn₂O₄ compoundcan be performed using any mixing techniques, however, mixing thatimproves the amount of contact between the LiMn₂O₄ compound and thelithium metal is preferred.

The lithium metal in one embodiment, can be added all at once. Inanother embodiment, the lithium is added in smaller increments, e.g. x/4or less. Such addition avoids distortion of the x-ray diffractionpattern, and allows the Li_(x)MnO₂ compound to maintain an x-raydiffraction (crystallinity) pattern similar to that of EMD.

The lithium metal used with embodiments of the present invention caninclude stabilized lithium metal powder (“SLMP”). For example, FMCCorporation produces a stabilized lithium metal powder under the nameLectro® Max Powder that may be used with embodiments of the presentinvention. Other lithium metal powders may also be used. For instance,U.S. Pat. No. 5,567,474 and U.S. Pat. No. 5,776,369, describe stabilizedlithium metal powders and processes for making such powders that can beused with the embodiments of the present invention.

Stabilized lithium metal powders allow the methods of embodiments of thepresent invention to be performed with increased safety. However,lithium metal powders that are not stabilized can also be used withembodiments of the present invention. In those embodiments wherenon-stabilized lithium metal or lithium metal powders are used,additional processes can be employed to improve the safety of thereactions. For example, the mixing of an LiMn₂O₄ compound with thenon-stabilized lithium metal or lithium metal powder can be performed inan inert atmosphere to inhibit undesired reactions of the lithium metalwith the atmosphere.

In other embodiments of the present invention, a doped Li_(x)MnO₂compound can be formed by mixing a doped LiMn₂O₄ compound with lithiummetal. The doped LiMn₂O₄ compounds can include LiMn₂O₄ compounds dopedwith dopants such as cobalt (Co), nickel (Ni), magnesium (Mg), titanium(Ti), zirconium (Zr), chromium (Cr), or other dopants used in theproduction of electrode materials for use with batteries andrechargeable lithium-ion batteries. The lithium metal is preferably astabilized lithium metal powder.

The mixing of lithium metal with LiMn₂O₄ or doped LiMn₂O₄ spinelcompounds can be performed in a ball mill or according to other mixingtechniques. In some embodiments, the mixing preferably includesenergetic mixing which increases the mixing of the compounds, improvingthe amount of contact between the LiMn₂O₄ compounds and the lithiummetal.

The mixing of lithium metal with LiMn₂O₄ can be performed with orwithout a solvent. If a solvent is used, the solvent is preferablycompatible with lithium such that the lithium metal does not react withthe solvent during the mixing. Solvents that can be used withembodiments of the present invention include, but are not limited to,acyclic and cyclic hydrocarbons, including n-hexane, n-heptane,cyclohexane, and the like; aromatic hydrocarbons such as toluene,xylene, isopropylbenzene (cumene), and the like; symmetrical,unsymmetrical, and cyclic ethers, including di-n-butyl ether, methylt-butyl ether, tetrahydrofuran, and the like.

In some embodiments of the present invention, the LiMn₂O₄ compounds canbe produced by calcining a mixture of at least one manganese oxide, atleast one lithium compound, and optionally at least one dopant in afiring step at a temperature between 400° C. and 900° C. The manganeseoxide compounds can include such compounds as Mn₂O₃, Mn₃O₄, electrolyticmanganese dioxide, and β-MnO₂, and the firing step can include multiplefiring steps.

In the calcining step, the mixture of source compounds is fired atbetween about 400° C. and about 900° C. Preferably, the mixture iscalcined using more than one firing step at firing temperature with thistemperature range. During calcinations, agglomeration of the spinelparticles is preferably prevented. For example, during a multiple stepfiring sequence, agglomeration can be prevented by firing the sourcecompounds in a fluid bed furnace or rotary calciner during at least aportion of the firing steps or by grinding the spinel material betweensteps.

The manganese oxide compounds produced in this manner can be formed intoLiMn₂O₄ compounds that can be used with embodiments of the presentinvention. In addition, other methods for forming lithium manganeseoxides may be used with embodiments of the present invention. Forinstance, the methods and compounds of U.S. Pat. Nos. 6,267,943;6,423,294; and 6,517,803 may be used with embodiments of the presentinvention.

The lithiated EMD materials formed according to embodiments of thepresent invention exhibit a capacity of about 150 mA·hr/g to about 160mA·hr/g when incorporated into an electrode. In addition, the lithiatedEMD materials of the present invention can be made cheaply because EMDcompounds are readily available and easily produced.

According to some embodiments of the present invention, the lithiatedEMD materials of the present invention can be used as low cost materialsfor forming electrodes for use with lithium ion batteries.

Embodiments of the invention also include batteries and electrodesformed from compounds and materials produced according to embodiments ofthe present invention. An electrode for use with a lithium ion batterycan be formed from the Li_(x)MnO₂ compounds or doped Li_(x)MnO₂compounds formed according to embodiments of the present invention. Inaddition, the lithiated EMD materials formed according to embodiments ofthe present invention can be used to form electrodes for use in lithiumion batteries. The Li_(x)MnO₂ compounds and lithiated EMD materialsformed according to embodiments of the present invention can be used toform anodes or cathodes for use in batteries and especially for use withrechargeable lithium ion batteries.

Having now described the invention, the same will be illustrated withreference to certain examples, which are included herein forillustration purposes only, and which are not intended to be limiting ofthe invention.

EXAMPLES Example 1

Lithium is added into a heat treated electrolytic manganese dioxide(“HEMD”). Electrolytic manganese dioxide available for Erachem-Comilogwas ground to reduce the particle size and heat treated at 400° C. for12 hours to obtain heat treated electrolytic manganese. The lithium isadded in small increments of 0.075 moles of Li per one mole of manganeseoxide. The addition is done in glove box at room temperature andstainless steel ball mill jar is used as a mixing vessel.

FIG. 1 shows the x-ray diffraction patters of HEMD with no lithium andthe various total addition amounts (0.30 moles Li to 0.58 moles Li).Comparison of the x-ray diffraction patterns demonstrates that thelithium can be added incrementally without distorting the structure ofthe HEMD to maintain the HEMD-like structure.

Example 2 and Comparative Example 1

Li_(0.3)MnO₂ is prepared by two ways. In Comparative Example 1, all 0.3moles of lithium to one mole manganese oxide are added at once. InExample 2, the lithium is added in increments of 0.075 moles lithium toone mole manganese oxide.

The x-ray diffraction pattern of FIG. 2 shows a well crystallinespinel-like structure for the Li_(0.3)MnO₂ of Comparative Example 1.This is contrasted to the x-ray diffraction pattern for Example 2 whichshows a pattern similar to that of the HEMD raw material sample andgraphically indicates very little distortion therefrom.

FIG. 3 shows electrochemical results. The Li_(0.3)MnO₂ of Example 2shows an increase of first charge efficiency from 45 percent to 93percent as compared to the one-step addition process of ComparativeExample 1. The voltage profile was sustained for over 10 cycles whichimplies no structural changes occurred. Such sustaining of the voltageprofile indicates such a material is a good candidate for 3Vrechargeable lithium batteries.

Examples 3 and 4 and Comparative Example 2

Li_(0.6)MnO₂ is prepared by three ways. In Comparative Example 2, all ofthe 0.6 moles of lithium to one mole of manganese oxide are added atonce. In Example 3, the 0.6 moles of lithium to one mole of manganeseoxide are added in increments of 0.15 moles. In Example 4, the lithiumis added in increments of 0.075 moles of lithium to one mole ofmanganese oxide.

The x-ray diffraction pattern for Comparative Example 2 in FIG. 4 showsa well-crystalline spinel-like structure for the Li_(0.6)MnO₂ but isdistorted as compared to the HEMD raw material sample. This iscontrasted to Examples 3 and 4 which show patterns similar to that ofthe HEMD raw material sample and indicates very little distortion.

FIG. 5 shows electrochemical results. The Li_(0.6)MnO₂ of Example 3shows an increase of first charge efficiency from 39 percent to 81percent as compared to the one-step addition process of ComparativeExample 2.

Having thus described certain embodiments of the present invention, itis to be understood that the invention defined by the appended claims isnot to be limited by particular details set forth in the abovedescription as many apparent variations thereof are possible withoutdeparting from the spirit or scope thereof as hereinafter claimed.

1. A method for forming Li_(x)MnO₂, comprising mixing an LiMn₂O₄compound with stabilized lithium metal powder in increments of onequarter of x or less to form Li_(x)MnO₂ where 0.2<x≦2 wherein thecrystalline structure of the LiMn₂O₄ compound is maintained.
 2. Themethod of claim 1, wherein the Li_(x)MnO₂ compound is doped with adopant selected from the group consisting of cobalt, nickel, titanium,zirconium, and chromium.
 3. The method of claim 1, wherein mixing theLi_(x)MnO₂ compound is formed by energetically mixing a LiMn₂O₄ compoundwith lithium metal in a ball mill.
 4. The method of claim 1, wherein theball mill is a high energy ball mill.
 5. The method of claim 3, whereinmixing the LiMn₂O₄ compound with stabilized lithium metal powder furthercomprises mixing the LiMn₂O₄ compound with the stabilized lithium metalpowder in the presence of a solvent.
 6. The method of claim 5, whereinthe solvent comprises a solvent selected from the group consisting ofacyclic hydrocarbons, cyclic hydrocarbons, aromatic hydrocarbons,symmetrical ethers, unsymmetrical ethers, and cyclic ethers.